Continuous-use molten metal inclusion sensor

ABSTRACT

A molten metal inclusion sensor intended for &#34;continuous&#34; use in the testing of steel, i.e. a useful life of at least about 30 minutes, comprises a probe which is detachably connected to a water-cooled support member (35). The probe comprises a tube (30) of heat resistant material, preferably silica, having an inner electrode (31) mounted on its interior wall and an outer electrode (32) mounted on its exterior wall, the molten metal entering the tube interior through an orifice (33) upon its immersion in the molten metal, whereupon the flow of metal with entrained inclusions is monitored by measuring the voltage between the electrodes (31, 32). The electrodes (31, 32) are preferably of graphite and are shaped to fit closely against the walls of the part of the tube (30) that is immersed in the metal and are of a material that retains enough mechanical strength to support the tube (30) as the metal is pumped into and out of interior, the metal remaining hot enough for this pumping to occur. The orifice (33) is contoured to produce streamline flow and the Reynolds number of the flow preferably is kept below 2000.

TECHNICAL FIELD

This invention relates to an apparatus for detecting the content ofinclusions in molten metal, such as precipitated secondary phaseparticles, drops of slag, and/or air bubbles, during refining thereof,all of which cause a discontinuity in the flow of current in the sensingzone and can therefore be sensed by measurement of this discontinuity.Hereinbelow, for convenience, all of these will be collectively referredto as "inclusions".

In general all such inclusions have a more or less deleterious effectupon the required technical properties of the metal, and it has becomemore and more essential to have accurate information as to their numberand sizes, in order to confirm that the metal is sufficiently "clean"for its intended purpose, and also to show whether the processesemployed are producing sufficiently "clean" metal.

INDUSTRIAL APPLICABILITY

The range of molten metals to which the present invention can be appliedis board and includes molten metals subjected to refining in steelmanufacture, aluminium refining, copper refining, titanium refining,magnesium refining, alloys of these metals, and the like. However, inthe following description, molten steel in steel manufacture will beused primarily as an example.

BACKGROUND ART

One prior art invention which relates to the present invention isdescribed in U.S. Pat. No. 4,555,662, issued November, 1985, this patentdisclosing a quantitative measurement method for inclusions, the methodnow being generally referred to as Liquid Metal Cleanliness Analysis(LiMCA for short). The LiMCA method and apparatus were originallydeveloped for detecting nonmetallic inclusions during aluminiumrefining, but its application to iron and steel refining has also beeninvestigated.

The LiMCA method is sometimes also referred to as the Electric SensingZone method (ESZ for short), the principle of the method being that whensuch an inclusion entrained in an electrically conductive fluid passesthrough an electrically-insulated orifice the electrical resistance ofthe fluid which is flowing through the orifice changes in proportion tothe volume of the particle. The instantaneous change in the resistanceis detected as a pulse in electrical potential between two electrodes onopposite sides of the orifice, and the number and size of the particlescan be directly measured in the following manner.

First, if the particles are assumed to be spherical and of diameter dand the orifice is assumed to be cylindrical of diameter D, then thechange R in the electrical resistance when a particle passes through theorifice is given by the following equation:

    ΔR=(4ρd.sup.3)/(πD.sup.4)                     (1)

Where ρ is the electrical resistivity of the fluid.

In actual practice, Equation (1) must be corrected by a correctionfactor F(d/D), which is given by the following equation:

    F(d/D)=[1-0.8(d/D).sup.3 ].sup.-1                          ( 2)

Thus, ΔR is actually expressed by the following equation:

    ΔR=((4ρd.sup.3)/(πD.sup.4)×[1-0.8(d/D).sup.3 ].sup.-1 ( 3)

If the electric current through the orifice is I, then the pulse V inthe electric potential when a particle of diameter d passes through theorifice is given by the following equation:

    ΔV=i(ΔR)                                       (4)

A previously-disclosed inclusion sensor probe which applies theabove-described principles and intended for "continuous" use with moltenmetal (e.g. for periods as long as about 30-40 minutes) comprises aninner first electrode supported inside a quartz tube and connected to awater-cooled support. An orifice is provided in a portion of the quartztube near to its lower end. The tube is mounted on the water-cooledsupport using a gasket to seal the joint between them. The necessaryouter second electrode consists of a rod separate from the probe andextending close to the orifice.

When a measurement is to be performed the inside of the hollowelectrode, which serves as a chamber to receive the molten metal, isevacuated and the molten metal is sucked inside through the orifice. Atthis time, the change in electric resistance between the inner and outerelectrodes is measured and amplified by conventional means, and thesizes and number of inclusions are determined. When the tube issufficiently full the negative pressure is replace by a positivepressure until the tube is empty and the cycle is repeated as many timesas possible until the tube must be replaced.

The above-described sensor probe and others are used to perform"continuous" measurement by the LiMCA method in order to detectinclusions in molten aluminium and determine particle sizedistributions. Molten aluminium has a relatively low melting temperatureof about 700° C., so there are a number of different materials availablefrom which the tube (heat resistant glass and quartz) and the electrodes(steel wire) can be made. However, the working temperatures of moltenmetal baths of metals like iron and titanium are much higher than foraluminium (above 1550° C.), and at such temperatures there areconsiderable problems with lack of resistance of the probe and theelectrodes to heat, so that it is difficult to employ these knownsensors. There have been some applications of sensors of this type formeasurement in the laboratory of inclusions of certain molteniron-silicon alloys having a temperature in the molten state of 1250° C.

In order to apply the LiMCA method to molten steel and the like, it isnecessary to solve the following problems.

REQUIREMENT FOR FLOW OF MOLTEN METAL INTO AND OUT OF PROBE BODY

At the temperatures at which the sensor must operate it is difficult tofind materials having the required properties of electrical insulationwhich are still sufficiently mechanically strong and are of sufficientlow cost to be commercially acceptable. Normally to try to overcome thisproblem one would take all possible measures to cool the probe and itscomponent part, but this is not possible with a sensor that is intendedto operate continuously, since the first metal to enter would quicklyfreeze and could not subsequently be removed during the part of thecycle when the sensor is emptied for re-use. Therefore at least the partof the probe body that receives the molten metal must always bemaintained at a temperature that is high enough that freezing, or evenpartial cooling for the metal to become too viscous, cannot occur, andit is this requirement that makes the provision of a satisfactory probeso difficult, and for which the present invention is a solution.

HEAT RESISTANCE OF A REFRACTORY PROBE BODY

When a probe is immersed in a molten metal with a high melting point,such as molten steel with a melting point of at least 1500° C., thetemperature reached by the electrically insulating probe body may exceedthe softening point of the material of which it is made. Therefore, whenthe molten metal is sucked inside the probe, and often before a singlecycle can be completed, the probe body ends up buckling or deforming andsubsequent measurement is impossible.

It is possible to make a probe body from a material such as boronnitride (BN) which has good resistance to high temperatures. However,such a probe body is extremely expensive (more than 10 times the cost ofa quartz tube), and is therefore too costly to be employed in routinecommercial operations.

MELTING DAMAGE OF THE INNER ELECTRODE AND POOR ELECTRICAL CONTACT DUE TOADHESION OF SLAG, ETC.

In the prior art methods of which we are aware rod-shaped electrodesmade of steel wire (or steel barstock), heat-resistant alloys, and thelike were employed for the inner electrode. However, when the hot metalenters the probe even the first time, the rod-shaped metal electrodeundergoes melting damage, and on the second and subsequent times theelectrode may have become so short that it is difficult for the surfaceof the molten metal to reach its tip so that electrical contact is notachieved and measurement cannot be performed.

In order to cope with this problem, attempts have been made to useelectrically conducting, heat-resistant materials such as graphite andzirconium boride (ZrB₂) for the inner electrode. However, duringmeasurement a slag layer which is caused by the nonmetallic inclusionsis formed on the surface of the electrode, and this leads to such poorelectrical connection as to make measurement impossible.

MAINTAINING AIRTIGHTNESS BETWEEN PROBE HEAD AND PROBE BODY

The gasket or O-ring which is normally inserted between the probe headand the probe body to seal the joint is of course made of aheat-resistant material. In the case of a relatively low melting pointmetal such as aluminum, there is a correspondingly less problem with thesealing ability of the gasket. However, in the case of a high meltingpoint metal such as molten steel, even if the probe head iswater-cooled, the gasket or the O-ring are quickly deteriorated by heatconduction from the probe body and/or heat radiation from the surface ofthe molten metal, and as a result it quickly becomes impossible tomaintain the airtightness of the inside of the probe during measurement.

In this case, not only does measurement become impossible due to theinability to such in or discharge molten metal, it also becomesdifficult to accurately determine the amount of molten metal which wassucked in or discharged, and accurate determination of the concentrationper unit volume of the particles being measured becomes impossible.

RESISTANCE TO MELTING OF THE HEAT-RESISTANT PROBE BODY

The outside of a heat-resistant probe body is corroded and subjected tomelting damage by contact with the slag or flux which normally coversthe surface of the molten metal and if holes are formed measurementbecomes impossible. In order to prevent this it has been attempted toform the entirety of those portions of the probe body which are immersedfrom a slag-resistant material, such as boron nitride. However, theseslag-resistant materials are expensive, so that with this proposal alsothe probe body becomes expensive and its cost makes it commerciallyuneconomical.

DISCLOSURE OF THE INVENTION

As a result of various investigations aimed at solving such problems,the present inventors have made the following invention.

In accordance with the present invention there is provided a moltenmetal inclusion sensor of the type which is immersed in a molten metaland detects inclusions in the molten metal by the electric sensing zonemethod, comprising a probe head and a probe supported by the probe headcharacterized in that the probe comprises a tube of electricallyinsulating material which constitutes a probe body, which tube issupported by the probe head and which is immersed in the molten metaland heated thereby, the tube having an orifice for the inflow andoutflow of molten metal formed in the part of the tube that is immersedin the metal, an inner electrode having the form of an electricallyconducting inner tube which is mounted on the inner wall of said tube,and an outer tube which is mounted on the outside of said, said innerand outer tubes providing physical support for at least the part of theprobe body tube that is immersed in the metal.

In accordance with the present invention there is provided a moltenmetal inclusion sensor of the type which is immersed in a molten metaland detects inclusions in the molten metal by the electric sensing zonemethod, comprising a probe head and a probe supported by the probe headcharacterized in that the probe comprises a tube of electricallyinsulating material which constitutes a probe body, which tube issupported by the probe head and which is immersed in the molten metaland heated thereby, the tube having an orifice for the inflow andoutflow of molten metal formed in the part of the tube that is immersedin the metal, an inner electrode having the form of an electricallyconducting inner tube which is mounted on the inner wall of said tube,and an outer tube which is mounted on the outside wall of said tube,said inner and outer tubes providing physical support for at least thepart of the probe body tube on which they are mounted and that in use isimmersed in the metal.

Preferably, the said outer tube is of electrically conducting materialand constitutes an outer electrode.

Preferably, the probe head that supports the probe body is water-cooled.

Preferably the material of the probe tube is silica and the material ofthe inner electrode or both the inner and outer electrodes is graphite.

MAINTAINING HEAT RESISTANCE OF THE PROBE BODY TUBE

Thus, the heat resistance of the probe body tube can be maintained orreplaced by using as the inner electrode a tube of a material havinghigh-temperature strength (i.e. one having a softening point temperaturewhich is higher than the temperature of the molten metal) inside theprobe body tube. The probe body is thereby supported against negativepressure during suction and positive pressure during evacuation, andeven if the heat-resistant material of the probe body tube softensappreciably, the tube still will not buckle.

MELTING OF THE INNER ELECTRODE INSIDE THE PROBE, AND POOR ELECTRICALCONTACT DUE TO ADHESION OF SLAG, ETC.

If the inner electrode is formed as a hollow, electrically conductingtube of heat-resistant material inserted inside the probe body and ismade to function as the inner wall of the probe body metal-receivingenclosure, it not only functions as an inner electrode but upon meltingthe electrode material remains with the electrode and damage isprevented as much as is possible. Furthermore, the surface area ofcontact with the molten metal is increased, so that poor electricalcontact is reduced. Also the structure of the probe body is simplified.

MAINTAINING AIRTIGHTNESS BETWEEN PROBE HEAD AND PROBE BODY

The following measures assist in counteracting against this problem.

(a) By inserting a heat insulating transverse member inside the probebody tube at the upper end of the inner electrode any sealing gasketand/or O-ring is insulated from heat radiation from the molten metalwhich flows into the probe body, and heat deterioration can thereby bereduced.

(b) Additionally or alternatively, if the probe head (and/or a probeholder which supports the probe body through a coupler between the probehead and the probe body) are water-cooled the effect on the gasketand/or O-ring of heat radiation from both inside and outside of theprobe body can be minimized.

RESISTANCE TO MELTING DAMAGE OF HEAT-RESISTANT PROBE HEAD

If the outside of those portions of a heat-resistant probe body whichmay contact molten slag or flux as the probe is inserted into the meltare protected with a slag-resistant material melting damage of the probebody from the outside can be effectively prevented. With this method,two different modes are conceivable.

(A) A separate outer electrode is used and a nonconducting outer tube ismounted on the probe body, or

(B) An outer tube made of an electrically conducting refractory ismounted on the probe body. In this case, the outer tube can be used asan outer electrode, and the heat resistance of the probe body isparticularly improved.

DESCRIPTION OF THE DRAWINGS

The prior art ESZ method and apparatus will now be described in moredetail, and a probe which is a preferred embodiment of the inventionwill now be described, by way of example, with reference to theaccompanying drawings, wherein:

FIG. 1(a) and 1(b) are illustrations which explain the principles ofinclusion detection by the ESZ method;

FIG. 2 is a longitudinal cross-sectional view of a continuousmeasurement prior art inclusion sensor probe which utilizes the ESZmethod, and employs a separate outer electrode;

FIG. 3 is a longitudinal cross-sectional view of an inclusion sensorprobe of the invention; and

FIG. 4 is a similar view of FIG. 2 of another prior art sensor probe.

DESCRIPTION OF PRIOR ART METHOD AND APPARATUS

FIG. 1a illustrates an electrically-insulated orifice 10 of diameter Dformed in a wall through which flows an electrically conductive fluid14, namely molten metal. Non-conductive inclusion particles 12 ofdiameter d that are entrained in the fluid and flow through the orificeeach give a respective resistance change and consequent electricpotential pulse ΔV illustrated by FIG. 1b. FIG. 2 shows the probe of aprior art inclusion sensor which applies these principles and of thetype having a separate outer electrode. In some cases, a level sensor ismounted inside the probe body.

A probe body 16 which is vertically supported by a water-cooled probehead 15 is constituted by an electrically insulating tube made ofquartz, for example, an orifice 17 being formed near to its tip. Arod-like inner electrode 18 passes through the probe head 15 and isinserted into the interior of the electrically insulating tube,extending to the vicinity of the orifice 17. The probe body 16 isconnected to the probe head 15 in an airtight manner by means of agasket 19. The inner electrode 18 is also mounted on the probe head 15in an airtight manner by means of an electrically insulating,heat-resistant gasket 20. The inside of the tube is connected to asuitable air supply and exhaust system through a pipe 21. When the probebody is immersed into molten metal air flows into and out of the tubethrough the orifice 17. An outer electrode 22 is disposed in a locationconfront the orifice 17.

The probe body 16 is immersed in the molten metal and then the exhaustsystem is operated to produce a vacuum inside the tube interior, causingflow of molten metal into the tube. The size and quantity of inclusionsin the metal are then measured based on the change in the electricalresistance between the inner and outer electrodes. As described abovethis prior art sensor is used to perform "continuous" measurement byLiMCA in order to detect and measure inclusions in molten aluminium.

BEST MODES OF CARRYING OUT THE INVENTION

In the sensor of the invention shown in FIG. 3, an inner cylindricalelectrode 31 and an outer cylindrical electrode 32 are electricallyinsulated from one another by an interposed, elongated tube 30 ofelectrically-insulating, heat-resistant material, in this embodiment aquartz tube, which constitutes the probe body. The lower portion of thetube 30 is provided with an orifice 33 in the same manner as in FIG. 2.When the probe is immersed in molten metal and a vacuum is applied tothe pipe 46, the molten metal is sucked into the tube 30 through theorifice 33. The tube 30 is supported by a water-cooled probe head 35 atits top end, a pressure-sealing gasket 34 being interposed between them.The probe is moved into and out of the metal by any suitable handlingmechanism (not shown) attached to the probe head 35. The inner and outerelectrodes 31 and 32 are respectively connected to electrode rods 36 and37. A cover member 40 for the upper end of the inner tube constitutes aheat-insulating shield member shielding the upper portion of the tubefrom heat from the molten metal in the lower portion.

The lower end of the inner electrode is provided with a shaped endportion 43 which fits snugly into the lower end of the tube, while theouter electrode is also shaped to fit snugly around the rounded outerend of the tube 30. The inner and outer electrodes are provided withrespective apertures 44 and 45 surrounding the orifice 33 and throughwhich the molten metal passes. The lower end of the tube which isinserted into the molten metal is therefore substantially entirelysandwiched, enclosed and supported between the two electrodes 31 and 32,and only the small portion adjacent the orifice is directly exposed tothe molten metal. The shield member 40 is provided with a bore 47through which the vacuum and pressure are applied to the interior ofmetal receiving chamber 48, and thus also ensures that metal cannotsplash and reach to the portion of the tube 30 that is not protected bythe internal electrode 31 and the cover 40.

The preferred material for the inner electrode 31 and theheat-insulating member 40 is graphite, which as well as beingelectrically conductive, retains enough mechanical strength at themolten metal temperature to be able to support the probe body againstcollapsing due to softening when molten metal is sucked in and forcedout. In addition, the inner electrode provides a large surface area ofcontact with the molten metal within the probe body and ensures goodelectrical contact.

The heat-insulating shield member 40 prevents the centre of the probehead 35 from overheating by radiation from the metal, and as a resultthe degradation and wear of the gasket 34 are at least substantiallyreduced. Therefore, good airtightness can be maintained between theprobe head 35 and the probe body 30 for the period for which the sensoris operative, and the suction and discharge of molten metal into andfrom the electrically insulating tube 30 is carried out smoothly. Inaddition, because good sealing is maintained the pressure inside theprobe body 30 can be measured accurately and it is possible toaccurately determine the amount of molten metal which is sucked in ordischarged, so that accurate measurement of the concentration ofparticles per unit volume can be performed.

The electrically conducting, heat-resistant inner electrode 31 istherefore used for the purpose of increasing the heat resistant strengthof the probe body, for preventing the adhesion of slag to the insides ofthe probe body, and for preventing melting damage to the innerelectrode. In addition, the probe head 35 is water-cooled in order tomeasure and maintain the airtightness between the probe head and theprobe body. The outer electrode 32 which is also made from anelectrically conducting, heat-resistant material also has resistance toslag erosion so it also functions as a slag protection layer. At thesame time, as it is co-extensive with the probe body 30, it helpsincrease the compactness of an inclusion sensor in accordance with thepresent invention.

The upper end of the tube 30 that is not immersed in the molten metal isprovided with an enclosing cylinder 42 of slag-protecting material, andthe prevention of splashes and the effect of heat radiation from thebath of this upper end is also improved by the outwardly extendingflange 41 of the outer cylinder 32, this flange effectively providing ademarcation between the part of the sensor that is immersed and the partthat always remains above the surface of the bath. The slag layer mayvary widely in thickness over a range of as much as 1-15 cm and atypical length for the tube 30 is 30-40 cm with an internal diametertypically of about 4-5 cm. The thickness of each of the inner and outerelectrodes is typically 4-10 mm, more usually about 6 mm.

The orifice (33) may be provided with a thin metal cover 49 of amaterial of lower melting point than the metal bath that is melted bythe molten metal once the probe is in the bath, the cover preventingentry of slag into the aperture 33 as it passes through the slag layerupon being first inserted into the bath. A suitable material isaluminium of thickness in the range 0.1-1.0 mm.

The size of the orifice 33 that is required can vary relatively widely,depending upon the metal whose cleanliness is being investigated and thenature of the inclusions therein. A minimum value typically is 200microns, but some steels are found to have inclusions measuring as muchas 250 microns, so that orifices as large as 1.2 mm may be required.Some of the inclusions found in steel, such as alumina andaluminosilicates, are known to have a tendency to adhere to refractorymaterials and it is important to prevent this from happening, since theymay accumulate at the orifice and at least partly bock it. One way of atleast reducing this effect is to shape the orifice so that both theentrance and the exit are smoothly rounded, thereby avoiding turbulenceand recirculation of the ingoing flow as much as possible; the choice ofthe material for the tube 30 will be affected by the ease ofeconomically producing such a contoured aperture. For example, with asilica tube it is found possible to produce the orifice economically byfirst drilling a hole using a watch-makers diamond drill and thenheating the edges of the hole with a micro-torch (oxy-acetylene) topartially fuse the silica and allow surface energy forces to shape it tothe required contour. The initial size of the hole is chosen to achievethe desired final size of orifice. With a long narrow tube, as is usedfor the probe, the entry can be shaped conveniently in this manner, butshaping the exit is more difficult and to facilitate this the orificemay be formed in a disc-shaped insert which is then mounted in asame-size aperture in the wall of the tube.

Another consideration in avoiding turbulence and consequent potentialfor clogging is to keep the Reynolds number of the flow to less thanabout 2000, since beyond this value the flow tends to become turbulenteven if the flow path is "streamlined" by the contouring of the orifice.The Reynolds number is given by the relation: ##EQU1## where ρ=densityof the fluid

U=mean flow velocity

d=diameter of orifice, and

μ=viscosity of the fluid It will be seen that ρ and μ are set by theprocess being employed and only U and d can be determined by the designof the probe and are intimately related to one another. The choice of dis somewhat restricted in dependence upon the size of the inclusions tobe measured, while the value of U can be controlled by the pressuredifference that is used to move the metal. If a large orifice is neededit may be necessary to use a relatively lower pressure differential toslow the flow to the required extent.

Materials-Electrodes

In order to establish good electrical contact between the molten metaland the electrodes, without which the LiMCA signals will be obscured ina background of electrical noise, it is important that as much wettingas possible be established along the current path between the twoelectrodes and the electric sensing zone is between. From the point ofview of choosing suitable electrodes, clearly they must not react withthe melt to form an electrically insulating oxide, or othernon-conducting layer.

Graphite is a much preferred material because of its cost but maypresent problems if the metal contains appreciable dissolved oxygen(e.g. above about 10 ppm) since there is then a tendency to produce CObubbles which can produce spurious signals, or even block the signalpath completely. One helpful technique is to employ a brief heavy"conditioning" current prior to the application of the test current, asis employed in the LiMCA technique, which is believed to help "burn-out"local areas of oxides or gas films in the orifice that otherwise produceincreased electrical resistance between the electrode and the melt. Inthe case of molten steel, particularly aluminium killed steel withconsequent low oxygen levels, graphite is a good choice in that it is areasonably good conductor of electricity and dissolves only slowly insteel. Further, in low carbon melts, graphite has a contact angle thatis a little less than 90°, i.e. it is slightly wetting which is againhelpful. The net effect of choosing graphite is an electrode whichpractically instantaneously establishes good contact. Further, asgraphite does not melt at typical steel-making temperatures (1500°-1650°C.) it is able to provide the required mechanical support to the probebody. Silica melts at about 1740° C. but is certainly somewhat softenedat steel bath operating temperatures, and therefore needs such support.

MATERIALS--PROBE BODY

A preferred electrical and thermal insulating material for the portionof the probe containing the orifice 33 is fused silica, despite itssoftening, because of its ready availability, lower cost and therelative ease of forming a contoured orifice. Moreover, silica ischemically attacked by iron and steel and it appears that the orifice iscleaned (reamed) by the flow of metal through it, so that good signalsare obtained. The use of a high initial conditioning current is alsouseful, and it is found that maximising the time of contact between thesilica and the steel also appears to improve performance, againstindicating against too rapid a flow rate.

Other suitable materials are boron nitride (BN) which has been employed,and titania (TiO₂), but these are both much more expensive than silicato the extent that the sensor may be commercially uneconomical. Boronnitride has been reported as having a contact angle with steel at 1550°C. of less than 50°, while titania has a contact angle with iron of 84°.Orifices are readily made in boron nitride but contouring of the edgesis more difficult; melts with high oxygen content (e.g. greater than1,000 ppm) should also be avoided since otherwise the boron nitride isquickly corroded.

The invention will now be explained in greater detail by means of thefollowing examples.

EXAMPLES

In a first example the concentration of inclusions in molten metal steelwas measured using an inclusion sensor of the invention having thestructure shown in FIG. 3, the sensor having an aluminium probe head 35,a steel electrode rods 36 and 37, a graphite inner electrode 31, agraphite outer electrode 32, an electrically insulating quartz tube 30,and a heat-resistant rubber gasket 34.

When measurement was performed molten metal was at 1550° C. and a slaglayer having a thickness of 10 mm was present atop the molten steel.Table 1 shows the composition of the molten steel, and Table 2 shows thecomposition of the molten slag.

                  TABLE 1                                                         ______________________________________                                        MOLTEN STEEL COMPOSITION (WT %)                                               C     Si      Mn      P     S     sol. Al                                                                              Total O.sub.2                        ______________________________________                                        0.05  0.12    0.60    0.018 0.006 0.045  0.0085                               ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        MOLTEN SLAG COMPOSITION (WT %)                                                CaO    SiO.sub.2 A1.sub.2 O.sub.3                                                                      MgO      MnO  FeO                                    ______________________________________                                        32.4   24.7      18.7    10.1     8.8  5.3                                    ______________________________________                                    

It was found that this probe of the invention could perform continuousmeasurement with no problems encountered while immersed in molten steelfor over 30 minutes.

COMPARATIVE EXAMPLE 1

In this example, a prior art type of inclusion sensor as illustrated byFIG. 2, having a probe body 16 made from a quartz tube, was used tomeasure the concentration of inclusions in molten steel in the samemanner and with the same conditions as in Example 1. The probe body 16was immersed in the molten steel and after approximately 3 minutes thepressure within the probe was reduced to 16.5 kPa, and the suction ofthe molten steel began. However, 15 seconds after the start of suctionthe probe body deformed and collapsed, due to the molten steeltemperature which exceeded its softening point, and due to the negativepressure within the probe, and measurement became impossible. The probehead 15 was water-cooled, and the gasket 19 was still sound after thetest.

COMPARATIVE EXAMPLE 2

In this example, the measurement of inclusions in molten steel wasperformed in the same manner as in Example 1, using the inclusion sensorshown in FIG. 4 and under the same conditions. This employed a compositeprobe body the lower part 50 which was immersed in the steel being madeof boron nitride (BN) with the upper part 51 that connected the lowerpart 50 to a coupler 57 made of silica. The inner electrode 52 was ofgraphite surrounded by a cylinder of mullite. The separate outerelectrode is not shown, but the inner electrode 52 was connected to anelectrode rod 53. The composite probe body 50, 51 was supported by aprobe head 54 through a coupler 57 which is equipped with an O-ring 56,the coupler being connected to and supported by a water-cooled probeholder 58. Reference 59 indicates a radiation shield member made of aheat-insulating material, such as a heat-resistant inorganic fiber, andwhich protects the O-ring 56 from heat from the molten steel. Referencenumeral 55 indicates the sensing orifice. The bath which was measuredcomprised the molten steel and the molten slag as described in Examples1 and 2. As in example 1, approximately 3 minutes after immersion of theprobe body the pressure within the probe body was reduced to 16.5 kPa tobegin the suction. When the level of the molten steel within the probebody reached the tip of the inner electrode the detection of LiMCAsignals commenced. Next, after the level of molten steel within theprobe body reached a prescribed level, the suction was stopped and, theinside of the probe was then pressurized with argon gas, so that themolten steel was discharged. After the discharge of had been nearlycompleted, molten steel was again sucked into the probe body, anddetection of signals due to inclusions was attempted a second time.However, even after the level of steel within the probe reached theinner electrode 52, the current through the signal detecting circuit wasextremely unstable, and the oscillation of the base line (noise) on anoscilloscope far exceeded the peak height of the signals due toinclusions, and thus the detection and measurement of signals wasimpossible. The reason for this was that when the molten steel wasdischarged, a slag layer and an inclusion layer adhered to the entiresurface of the small-diameter inner electrode, and the conductiveity ofthe electrode surface was greatly reduced.

COMPARATIVE EXAMPLE 3

In this example, Example 1 was repeated except that the probe which wasemployed was not equipped with a water-cooled probe head or a radiationshield. The molten bath which was measured comprised the molten steeland slag described in Tables 1 and 2 at 1550° C. The probe body wasimmersed in the molten steel for approximately 3 minutes, after whichthe pressure inside the probe was reduced to 16.5 kPa and the suctionand detection and measurement of LIMCA signals began. Three minutesafter the start of measurement it became difficult to suck or dischargethe molten steel, and the detection and measurement of signals becameimpossible. The reason for this was that the O-ring which was made ofheat-resistant rubber had burned due to radiation and conduction of heatfrom the molten steel, and the airtightness of the inside of the probecould not be maintained.

COMPARATIVE EXAMPLE 4

In this example Example 1 was repeated, but the probe body which wasemployed was not equipped with an outer tube and an outer electrode wasemployed. As with example 3, the bath comprised the molten steel andslag described in Tables 1 and 2 at 1550° C. Approximately three minutesafter the probe body was immersed in the molten steel the inside of theprobe was reduced to a pressure of 16.5 kPa and the suction anddetection and measurement of LiMCA signals was begun. However, giveminutes after the start of measurement the portion of the quartz tubeconstituting the probe body which contacted the molten slag sufferedmelting damage and holes were formed therein, so that subsequentmeasurement was impossible.

By way of summarizing the above results, the usable life of the probesin molten steel and their unit costs are compared in Table 3. Thepresent invention is not particularly inexpensive from the standpoint ofmanufacturing costs, but it can be seen that is is overwhelminglysuperior in its useful life.

                  TABLE 3                                                         ______________________________________                                                     Length of Normal                                                              Operation from Start                                                                          Cost per                                         Probe        of Suction of Molten                                                                          Probe *                                          Type         Steel (minutes) (Index)                                          ______________________________________                                        Present      At least 30 minutes                                                                           100                                              Invention                                                                     Comparative  15 seconds      75                                               Example 1                                                                     Comparative  At most 2 minutes                                                                             600                                              Example 2    (Only one measurement                                                         possible)                                                        Comparative  3 minutes       95                                               Example 3                                                                     Comparative  5 minutes       95                                               Example 4                                                                     ______________________________________                                         * Note:                                                                       Does not include probe holder or probe head.                             

It will be seen that, as described above, an inclusion sensor inaccordance with the present invention can perform continuous measurementof inclusions in a molten metal such as molten steel which has a highmelting point, and as it can perform continuous measurement for over 30minutes, it can be said to be a superior inclusion sensor for practicaluse.

    ______________________________________                                        Index of Reference Signs                                                      ______________________________________                                        10         Prior Art - insulated orifice                                      12         Prior Art - non-conducting particles                               14         Prior Art - electrically-conducting fluid                          15         Prior Art - water-cooled probe head                                16         Prior Art - probe body                                             17         Prior Art - orifice                                                18         Prior Art - inner electrode                                        19         Prior Art - tube/head gasket                                       20         Prior Art - head/electrode gasket                                  21         Prior Art - pressure/exhaust pipe                                  22         Prior Art - separate outer electrode                               30         quartz tube                                                        31         inner electrode                                                    32         outer electrode                                                    33         orifice                                                            34         tube/head gasket                                                   35         water-cooled probe head                                            36         inner electrode rod                                                37         outer electrode rod                                                38         head electrode gasket                                              40         heat insulating shield member                                      41         heat shielding flange                                              42         slag protecting envelope                                           43         inner electrode end                                                44         inner electrode aperture                                           45         outer electrode aperture                                           46         pipe to pressure/vacuum source                                     47         bore for pressure/vacuum passage                                   48         metal receiving chamber                                            49         orifice cover                                                      50         Prior Art - probe body lower part                                  51         Prior Art - probe body upper part                                  52         Prior Art - inner electrode                                        53         Prior Art - inner electrode rod                                    54         Prior Art - probe head                                             55         Prior Art - orifice                                                56         Prior Art - O-ring                                                 57         Prior Art - coupler                                                58         Prior Art - water-cooled probe holder                              59         Prior Art - radiation shield                                       ______________________________________                                    

We claim:
 1. A molten metal inclusion sensor of the type which isimmersed in molten metal and detects inclusions in the molten metal bythe electric sensing zone method, the sensor comprising:a probe head anda probe body supported by the probe head, the probe body having an upperend and a lower end so that the probe body is movable for immersion ofsaid lower end in the molten metal by which the probe body is heated;the probe body comprising: an elongated insulating tue of electricallyinsulating material that engages the probe head for its support therebyand resultant support of the probe body by the probe head, theinsulating tube having an upper end and a lower end corresponding to theupper end and lower end of the probe body, respectively; an innerelectrode having the form of an electrically conducting inner tubemounted on an inner wall of the insulating tube so as to extend from thelower end of the insulating tube to a position intermediate the lengthof the insulating tube; the inner tube electrode comprising ametal-receiving chamber having at an upper end a transversely extending,heat-insulating cover member inhibiting movement of molten metal to theupper end of the insulating tue and shielding the upper end of theinsulating tube from the heat of metal in the metal-receiving chamber;an outer electrode having the form of an electrically conducting outertue mounted on an outer wall of the insulating tube so as to extend fromthe lower end of the insulating tube to the position intermediate thelength of the insulating tube and above the molten metal when the probebody is inserted in the molten metal; the outer tube electrode having atan upper end a circumferential shield shielding the upper end of theinsulating tube from heat radiated from the molten metal; said inner andouter tube electrodes providing physical support for at least the partof the insulating tube on which they are mounted being immersed in themetal during use; and the insulating tube and the inner and outerelectrodes including registering orifices for the inflow and outflow ofmetal between the metal-receiving chamber and the molten metal.
 2. Asensor as claimed in claim 1, wherein the probe head (35) that supportsthe probe body is water-cooled.
 3. A sensor as claimed in claim 1,wherein the insulating tube (30) is comprised of silica and the innerand outer electrodes (31, 32) are comprised of graphite.
 4. A sensor asclaimed in claim 1, further comprising a cylinder (42) of slagprotecting material disposed around the upper part of the probe bodyabove the outer tube (32) to shield the probe body from slag on thesurface of the molten metal when the probe body is inserted into themolten metal.
 5. A sensor as claimed in claim 1, wherein the probe isprovided with an external cover member (49) covering the orifice (33)comprised of a material having a lower melting point than thetemperature molten metal, the cover member (49) permitting the probe tobe passed through a layer of slag on the surface of the molten metalwithout entry of slag into the metal-receiving chamber (48).
 6. A sensoras claimed in claim 1, wherein the orifice (33) has a contoured profileand entrance and exit openings smoothly rounded to reduce turbulence offlow therethrough.
 7. A sensor as claimed in claim 1, wherein the rateof flow of the molten metal into the probe is such that the Reynoldsnumber is less than 2,000.